Stand down after head of state breaks rule (6)
I believe the answer is:
resign
'stand down' is the definition.
(I've seen this before)
'after head of state breaks rule' is the wordplay.
'after head of state' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'breaks' means one lot of letters goes inside another (inserted letters break into the word).
'rule' becomes 'reign' (synonyms).
's' going inside 'reign' is 'RESIGN'.
(Other definitions for resign that I've seen before include "withdraw" , "Leave office voluntarily" , "Give up one's job" , "Give up office" , "Step down" .)